Spread Spectrum Modulation Scheme for Multilevel Inverters using Vector Quantized Sigma Delta Modulation

A generalized Space Vector based Spread Spectrum Modulation scheme for multilevel inverters is proposed in this paper. The proposed space vector based scheme is based on the view that a multilevel converter could be interpreted as an oversampling analog-to-digital converter. As the switching frequency of sigma delta modulator in the oversampling analog-to-digital converter varies randomly, the proposed scheme has spread spectrum characteristics. The principle of Vector Quantization is utilized for quantizing the instantaneous reference voltage space vector in the sigma delta converter. The switching vectors of the inverter are naturally selected without sector identification, ensuring optimum switching sequence under all conditions including the over-modulation region. To avoid fractional arithmetic, sixty degree coordinate system is used to represent the space vector in this paper. The proposed scheme is implemented for six-level inverter, driving 2-HP three phase induction motor and experimental results are presented. Experimental results of the proposed scheme are compared with Space Vector PWM scheme and Random PWM scheme.